1. Field of the Invention
This invention relates to a color toner used in a developer for developing an electrostatic image in image forming processes such as electrophotography, electrostatic recording and electrostatic printing. It also relates to a process for its production.
2. Related Background Art
It is conventionally known to form an image on the surface of a photoconductive material by an electrostatic means and develop the image.
A large number of methods are known, as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 42-23910 and No. 43-24748 and so forth. In general, an electrostatic latent image is formed on a photo-sensitive member, utilizing a photoconductive material and various means, and subsequently a toner is caused to adhere onto the latent image to form a toner image corresponding to the electrostatic latent image.
Next, the toner image is transferred to an image holding medium such as paper if necessary, followed by fixing by the action of heat, pressure, heat and pressure, or solvent vapor. A copy is thus obtained. In a case where the process comprises a toner-image transfer step, the process is usually provided with the step of removing the toner remaining on the photosensitive member.
As developing methods by which the electrostatic latent image is formed into a visible image by the use of a toner, known methods can be exemplified by the powder cloud development as disclosed in U.S. Pat. No. 2,221,776, the cascade development as disclosed in U.S. Pat. No. 2,618,552, the magnetic brush development as disclosed in U.S. Pat. No. 2,874,063, and a method in which a conductive magnetic toner is used, as disclosed in U.S. Pat. No. 3,909,258.
As toners used in these developing methods, a fine powder obtained by mixing and dispersing a colorant in a thermoplastic resin has been commonly used. The thermoplastic resin most commonly includes polystyrene resins. Besides, polyester resins, epoxy resins, acrylic resins and urethane resins are also used. As the colorant, carbon black is most widely used in the case of non-magnetic toners. In the case of magnetic toners, black magnetic powder of an iron oxide type are widely used. In a system in which a two-component type developer is used, the toner is usually used in a mixture with carrier particles such as glass beads, iron powder and ferrite powder.
The toner image finally formed on a copy image holding medium such as paper is fixed onto the image holding medium by the action of heat, pressure or heat and pressure. In this fixing, the step of fixing by heat has been hitherto widely used.
In recent years, a rapid progress is being made in copying machines from monochromatic copying to full-color copying, and researches are made on two-color copying machines or full-color copying machines, which have been put into practical use. For example, Journal of Electrophotographic Society, Vol. 22, No. 1 (1983) and Journal of Electrophotographic Society, Vol. 25, No. 1, p.52 (1983) make reports relating to color reproduction and gradation reproduction.
Images formed by full-color electrophotography presently put into practical use, however, are not necessarily satisfactory for those who are accustomed to seeing color pictures that are by no means immediately compared with the actual object or original and also processed more beautifully than the actual object or original, as in television pictures, photographs and color prints.
In the aforesaid development of electrostatic latent images, the toner is blended with a carrier formed of relatively large particles and is used as a two-component type developer for electrophotography. The composition of the toner and the carrier is selected so that as a result of their mutual contact friction the toner can have a polarity reverse to the charges present on the photoconductive layer. As a result of contact friction between the two, the carrier electrostatically attracts the toner to its particle surfaces to transport the toner through a developing assembly and also feed the toner onto the photoconductive layer.
When, however, copies are continuously taken on a large number of copy sheets by an electrophotographic copying apparatus using such a two-component type developer, although sharp images with a good image quality can be obtained at the initial stage, fog may greatly occur and edge effect may seriously occur after copies have been taken on several tens of thousands of sheets, tending to result in images having poor gradation and sharpness.
In color copying carried out using toners with chromatic colors, continuous gradation is an important factor that influences image quality, and the edge effect that stresses only margins of images, occurring after copies have been taken on a large number of copy sheets, greatly damages the gradation of images. For example, quasi-contours due to the edge effect are formed in the vicinity of actual contours, resulting in a poor color reproducibility and copy reproducibility in color copying. Image area used in conventional black and white copying is 10% or less, where original images are almost held by line images as in letters, documents, reports and so forth. On the other hand, in the case of color copying, image area is at least 20%, and images are held by gradational solid images at a considerable occupancy as in photographs, catalogues, maps, pictures, etc.
When copies are continuously taken using such originals having a large image area, reproductions with a high image density can be obtained at the initial stage in usual instances, but the feeding of toner to the two-component type developer may become gradually insufficient to cause a decrease in density, or the toner being fed and the carrier may mix in an insufficient state, where the toner may have an insufficient triboelectricity to cause fog or cause a local increase or decrease in toner concentration (which indicates toner-carrier mixing ratio) on the developing sleeve, tending to result in blurred images or non-uniform in-image density. This tendency becomes more remarkable when the toner has a smaller particle diameter.
Such under-development and fog are presumed to be caused by an excessively low toner concentration in developer or a poop rise for rapid triboelectric charging between the toner being fed and the carrier contained in the two-component type developer, where any insufficiently charged toner thereby produced participates in development. It is necessary for developers having color toners to have the ability to always output images with a good image quality in the continuous copying of originals having a large image area. To deal with originals having a large image area and requiring a very large toner consumption, efforts have hitherto been made for improvements of developing apparatus than improvements of developers themselves. It has been attempted to increase the peripheral speed of a developing sleeve or allow a developing sleeve to have a larger diameter so that the developing sleeve can be brought into contact with electrostatic latent images more many times.
Such measures can be effective for improving developability, but tend to result in a decrease in the lifetime of apparatus because of an in-machine contamination due to toner scatter from developing assemblies or because of an overload on the drive members of developing assemblies. In some instances, measures are also taken in which developers are put in developing assemblies in large quantities in order to compensate the insufficiency of developability of the developers. Such measures, however, cause an increase in weight of copying machines, a cost increase due to the apparatus that must be made larger in size and an overload on the drive members of developing assemblies, and are not preferable.
For the purpose of improving image quality, several developers are proposed. For example, Japanese Patent Application Laid-open No. 51-3244 discloses a non-magnetic toner in which its particle size distribution is controlled so that the image quality can be improved. This toner is mainly composed of toner particles having a particle diameter of 8 to 12 .mu.m, which are relatively coarse. According to studies made by the present inventors, it is difficult to "lay" the toner with such a particle diameter onto latent images in a uniform and dense state, and also the toner, as having the feature that particles with a particle diameter of 5 .mu.m or smaller are in an amount of not more than 30% by number and particles with a particle diameter of 20 .mu.m or larger are in an amount of not more than 5% by number, tends to cause a lowering of uniformity because of the broadness of its particle size distribution. In order to form sharp images by the use of the toner comprised of such relatively coarse toner particles and having a broad particle size distribution, the toner particles must be thickly overlaid so that any spaces between toner particles can be filled up to increase apparent image density. This brings about the problem of an increase in the consumption of toner necessary to attain a given image density.
Japanese Patent Application Laid-open No. 54-72054 discloses a non-magnetic toner having a sharper particle size distribution than the above toner. It, however, contains medium-size particles with a size as large as 8.5 to 11.5 .mu.m, and has room for further improvement for a toner with a high resolution.
Japanese Patent Application Laid-open No. 58-129437 discloses a non-magnetic toner in which an average particle diameter is 6 to 10 .mu.m and particles with a particle diameter of 5 to 8 .mu.m are present in the greatest number. This toner, however, contains particles with a particle diameter of 5 .mu.m or smaller in an amount as small as 15% by number, and tends to form images lacking in sharpness.
Japanese Patent Application Laid-open No. 2-222966 discloses a toner containing toner particles with a particle diameter of 5 .mu.m or smaller in an amount of 15 to 40% by number. This has brought about a considerable improvement in image quality, but it is sought to achieve a more improved image quality.
Japanese Patent Application Laid-open No. 2-877 discloses a toner containing toner particles with a particle diameter of 5 .mu.m or smaller in an amount of 17 to 60% by number. This has certainly brought about stable image quality and image density. However, if the toner has an extremely low coloring power, images with a poor highlight reproduction and fine-line reproduction are formed however properly the particle size distribution of the toner is controlled. In addition, the problem of density insufficiency occurs in solid areas. If on the other hand the toner has an excessively high coloring power, the problem of coarse images or fog also tends to occur in highlight areas. In order to stably obtain high-quality images, as stated above, not only the particle size distribution of toners but also the coloring power of toners are important factors.